In general, a chain or belt transmission device for valve timing in an internal combustion engine, or for transmitting rotational power in another drive mechanism, includes a chain or belt, which transmits power from a driving sprocket or pulley to one or more driven sprockets or pulleys. The transmission typically includes a pivotally mounted, movable, synthetic resin guide, which cooperates with a tensioner as a tensioner lever, and a fixed synthetic resin guide used as a guide rail for the chain or belt. The movable guide and the fixed guide are attached to a frame of the engine or other drive mechanism by suitable pins, or by bolts or similar mountings. The guides make sliding contact with the chain or belt to apply appropriate tension thereto, and also prevent widthwise vibration and widthwise movement of the chain or belt, which might cause it to run off the guides.
FIGS. 8 and 9 depict a well known, conventional sliding contact guide 500. The guide comprises a synthetic resin guide body 510, which includes a shoe 511 having a front surface on which a traveling chain slides and a plate-receiving portion 512 on the back side of the shoe. The plate-receiving portion is composed of a pair of walls 512a provided on the back of the shoe 511 and extending in the longitudinal direction of the guide. These walls are spaced from each other to define between them a longitudinally extending slot, which opens in a direction facing away from the shoe. A metallic reinforcing plate 520 is fitted between the slot walls 512a as depicted, for example, in Japanese patent application No. 2000-322380.
As shown in FIGS. 8 and 9, the reinforcing plate has a hole 521 for receiving a pin or bolt on which the guide is pivoted, a tensioner-contacting portion 522, and a pair of locking holes 523. The synthetic resin guide body 510 is a complicated structure, comprising a shoe 511, on which a chain slides, and a slotted plate-receiving portion composed of walls 512a, a mounting hole 512b, a boss 512c, outer reinforcing ribs 512d for strengthening the guide, a tensioner contacting portion 512e, and tongues 512f for locking engagement with the holes 523 in the reinforcing plate 520.
As illustrated in FIG. 10, in the molding process, the synthetic resin guide body 510 becomes warped through thermal shrinkage due to local variations in the rate of cooling. Warpage takes place in the directions depicted by arrows Y, and results in the formation of gaps S, as shown in FIG. 11, between the walls 512a of the slot and the reinforcing plate 520. As shown in FIG. 11, the width of these gaps increases so that the gaps are wider near the opening of the slot.
The mold draft, provided in order to facilitate removal of the finished guide body from the mold, contributes to the divergence of the gaps S. The gaps cause the metallic reinforcing plate 520 to fit loosely in the guide body 510, and shock noise occurs due to wobbling between the guide body 510 and the reinforcing plate 520 as a chain travels over the guide. The wobbling movement of the reinforcing plate and guide body leads to a significant reduction in the life of the guide.
If the slot gap S is made small in order to avoid the above-described problems, it becomes more difficult to insert the metallic reinforcing plate 520 into the guide body 510, and assembly of the guide becomes more difficult and time-consuming. The result is a significant reduction in productivity.
The slot gap in the guide body 510, and the thickness of the reinforcing plate 520 are ideally the same size. However, to mold these parts in the same size is difficult. For example, if the slot gap S is 0.1 mm, the metallic reinforcing plate 520 wobbles. On the other hand, if the thickness of the reinforcing plate 520 is 0.1 mm larger than the width of the slot the reinforcing plate either cannot be inserted into the guide body, or can be inserted only by the use of an excessive inserting force. Thus, excessive accuracy in molding, and very close dimensional tolerances were required.
Accordingly, objects of the invention are to solve the above-described problems of the prior art, and to provide a synthetic resin guide for a flexible power transmission medium, which can suppress shock noise due to the wobbling, and which can be produced inexpensively and with high productivity, without the need to maintain close dimensional tolerances in molding.